How can a father be helped to stop crying continuously over the death of his son, especially since he considers crying a mercy?

The calamity of death is inescapable, and grieving and weeping over the death of a relative, especially a son, is permissible if it is customary and not accompanied by wailing or discontent. The Prophet, peace and blessings be upon him, wept over his son Ibrahim and said: "Indeed, the eye sheds tears, and the heart grieves, but we do not say except what pleases our Lord. And indeed, we are saddened by your separation, O Ibrahim."

Weeping over the deceased is a mercy. The meaning of "this is mercy" is that the tear is an effect of mercy, meaning that the tears that flow from the heart due to grief, without deliberate intention or summoning, are not blameworthy. What is forbidden is panic and impatience.

As for the blameworthy weeping, it is that which involves wailing or lamenting, or which a person intentionally induces and exaggerates, indicating panic and impatience.

What helps one to be patient is to remind the father of the reward of the patient, and that Allah will pay the patient their reward in full without reckoning, and that their reward is better for them than their son remaining alive.

You must help your father by sitting with him, talking to him, and not leaving him alone. Take him to the mosque or visit relatives so that he can overcome his grief.